Please refer to FIG. 1, which is the schematic diagram showing a conventional projector with the light source of LEDs. The power of the conventional projector with the LED light source normally comes from the household power supply, such as the alternating current (AC) from the electric outlet on the wall, where the AC is rectified and transformed by the power supply 2 so that the electrical power can be used by the components inside the projector. Only the issues of the power supply for LED are discussed here. That is, only the issues regarding the power transmission from the power supply 2 to the driver 3 are discussed, and the other parts will not be considered. The power supply 2 usually has the function of transforming the voltage, usually reducing the voltage, and sometimes the function of rectifying or stabilizing the voltage. The AC from the outlet on the wall is outputted to the power supply 2. After the conversion and transformation, the power supply 2 output a supply voltage v1 to the drive 3. The drive 3 is responsible for driving the LED 4, and outputs a drive current A to the LED 4. Accordingly, the light source of the projector can operate properly.
However, some deviations occur during the manufacture of LEDs, so the operating voltage for each LED is not identical, but is somewhat higher or lower. Therefore, in order to cover the whole operating voltage from the lowest to the highest operating voltages, the power supply 2 outputs a higher supply voltage v1 than the average required voltage to the driver 3 in the conventional technique. Usually, the supply voltage v1 is possibly higher than the acceptable voltage by the LED by about one third of the acceptable voltage. Therefore, when the driver 3 generates the constant current, and outputs the driving current A to the LED 4, there will be an operating voltage v2 for the LED 4, and the difference between the operating voltage v2 and the supply voltage 1 exists. At this moment, the driver 3 absorbs the energy of this voltage difference and generates the heat for this over voltage. The negative effects are the generation of high heat and the electricity wastage.
For solving the above mentioned problems, the inventors had done a lot of efforts in the analyses, researches and developments, and finally developed the system and the method for controlling the power of LEDs.